Hi Greg,
interesting news. So for 1.6 MHz no more negative voltage at the gates and 50% duty cycle. This looks fine.
So something was caused by the switched drain voltage / current. But how came this into the gate circuit, by the chanel to gate capacity??? Or is there a wrong coupling between the drain and the gate circuit??? Questions but no answer.
But for 28 MHz it's terrible, both with all FET and with two FET removed. Nothing to be seen what looks like a 28 MHz signal.
Going back to your post from Jul 3th. The drain voltage was going up to around 61V. Assuming the supply voltage was 12 V this was
5 times of the supply!!! Checking my QMX the drain voltage was in the region of the supply voltage. Your high value looks like produced by a resonant circuit with a high real part of the impedance, not near 50 ohm.
Could you check
- the region of the BS170 for unwanted things,
- T501 and the area around,
- the PIN diode system of the LPF section,
- the G, S and D voltage at Q508 for RX and TX,
- the frequency response of your oscilloscope with the actual probe (e. g. checking a fast clock signal from the QMX).
For No 1 and 2 it would be fine to show detailed photos. For No 3 you will find information reading this
document from the QRP Labs Wiki (section PIN diode switching). It's for QMX but simple to expand for QMX+.
It would be also fine to remove all BS170 and check the signals from IC503 for different bands.
The step next is a visual check of the capacitors from "+12V" to GND and from VCC to GND.
To get a better impression of your system:
- which power supply do you use?
- which dummy load do you use?
Sorry for this lot of work.
73, Ludwig
